The present invention relates to an optical information recording medium and a method of recording and reproducing information on and from an optical information recording medium. More specifically, the present invention relates to an optical information recording medium which can be utilized as a write-once medium capable of realizing a high transfer rate and to a method of recording and reproducing information on and from an optical information recording medium.
Not only computer information but also information such as sound, static images or dynamic images are recently going into a digital form, and the amount of information handled by computers has been extremely increasing. Accompanied by this, information recording media are demanded that are capable of recording and reproducing large quantities of data with high density at high speed. In response to this demand, optical information recording media including DVD-R, DVD-RAM and DVD-RW are being produced. Moreover, because improvements in the processing speed of CPU's and in the maintenance and development of peripheral equipment and software have further progressed, enormous quantities of image information and audio signals are readily becoming freely handled today. Therefore, large-capacity high-speed optical information recording media have been increasingly demanded.
As optical information recording media optical disk which performs recording and reproduction of information by irradiation of a laser beam, two types of optical information recording media, a write-once medium and a rewritable medium, are well known. The write-once medium allows data to be recorded once but not overwritten, and the rewritable medium allows data to be overwritten for multiple times. Since it is impossible to overwrite the information recorded on the write-once medium, the write-once medium is more suitable for recording official documents and the like, preventing the information from being falsified.
There are two types of recording materials widely used for the write-once medium, an organic dye and a phase-change recording materials. An example of the organic dye employed as the recording material is a photosensitive organic dye including benzophenone series dyes, phthalocyanine series dyes, naphthalocyanine series dyes and cyanine series dyes. In the write-once medium which employs the phase-change material, a laser beam irradiates a Te-Ox film, a Te-Ox-Pd film or the like to cause a phase change, and a reflectance change accompanied by this phase change is detected to reproduce information (e.g., refer to Japanese Unexamined Patent Publication Number 61-168151).
To increase the speed demanded for the optical information recording medium, a conventional write-once medium has the following problem: for example, in the case where an organic dye is used as the recording material, recording sensitivity is likely to be insufficient when high-speed recording is performed by increasing a linear velocity of the medium. Thus, it is difficult to achieve a high transfer rate. In addition, the conventional write-once medium has another problem that it is hard to design an organic dye for recording/reproducing light with a short wavelength.
Moreover, in the case of a write-once phase-change recording film which employs a Te-Ox film or a Te-Ox-Pd film, an amorphous recording layer is irradiated with a laser beam to increase the temperature of the recording layer to a range from the crystallization temperature to the melting point, thereby growing crystal grains. Accordingly, crystal recording marks are formed. However, this phase-change process requires a long time to complete the crystallization, and therefore it is difficult to achieve a high transfer rate.
By contrast, in a rewritable medium which employs a phase-change material, a crystalline recording layer formed by, for example, Te—Ge series, As—Te—Ge series, In—Sb—Te and Ga—Sb—Te, is irradiated with a high-power laser beam to fuse. Thereafter, the recording layer is rapidly cooled down from the fused state to form amorphous recording marks. Thus, since the recording marks are formed by changing the phase of the recording layer, from crystalline to amorphous, it takes a relatively short time to form the marks compared with the write-once phase-change recording layer. Therefore, a high transfer rate can be possibility achieved. Furthermore, this crystalline recording layer absorbs the laser beam in a moment and reaches the melting point. Hence, the crystalline recording layer has an advantage over the write-once medium in which an organic dye is decomposed upon recording, that the crystalline recording layer is highly sensitive and the recording power thereof does not largely depend on a recording linear velocity.
In U.S. Pat. No. 6,242,157 of Tominaga et al., there is disclosed a phase change optical recording medium having on a substrate a recording layer consisting essentially of a Sb base thin film and a reactive thin film. The Sb base thin film is formed by depositing a Sb base material containing at least 95 at % of Sb to a thickness of 70–150 Å. The reactive thin film is formed of a material which forms a phase change recording material when mixed with Sb. The reactive thin film is typically formed of an In—Ag—Te or Ge—Te material. Stable write/read characteristics are accomplished at the first overwriting, initializing operation is eliminated, and rewriting is impossible at the same linear velocity as recording.
Under these circumstances, we have been exploring the possibilities for applying the phase-change (crystalline/amorphous) material used in the rewritable medium to the write-once medium. We focused on a phenomenon where it is hard to perform direct overwriting on a rewritable medium of a film incident light type rewritable medium, having the construction of a 1.1 mm-thick substrate, an Ag reflective alloy film, a ZnS—SiO2 protective layer, a GeSbTe series recording film, a ZnS—SiO2 protective layer and a 0.1 mm-thick cover layer when information is recorded and reproduced by use of a blue laser beam with a wavelength of 405 nm. This phenomenon arises because a spot size of the blue laser beam is smaller than that of a red laser beam and the heat absorptance of the amorphous recording marks is larger than that of the crystalline recording marks. Accordingly, when the amorphous recording marks are irradiated with the blue laser beam, the time required to crystallize the amorphous marks is shorter than that of the red laser beam. Therefore, the amorphous marks are incapable of being completely crystallized, or lengths of the recording marks vary between when information is overwritten on the amorphous recording marks and when information is overwritten in the crystalline spaces.
However, it is difficult to utilize this phase-change (crystalline to amorphous) recording material for the write-once medium because of the following reason. Amorphous recording marks are formed on a crystalline GeSbTe recording film by use of a laser beam. Thereafter, these amorphous recording marks area irradiated with a low-power laser beam for two to three times, and the amorphous recording marks are crystallized. As a result, recorded information is completely erased, and it becomes possible to overwrite new information. Thus, even when information is recorded and reproduced by use of a blue laser with a wavelength of 405 nm, the recording material cannot be utilized for the write-once medium.